The present invention relates to the field of centrifugal removal of fluids from solid fines such as ore slurries, industrial wastes, coal, and the like. More particularly the present invention relates to an improved method of operating batch-type centrifugal fine solids drying systems. In even greater particularity the present invention may be described as an improved method of operating batch-type centrifugal fine-solid drying systems by stabilizing a gimbal-mounted shaft and bowl combination under high speed cut-out and loading conditions, with said drying system utilizing very high speed rotation to achieve a surface moisture content of less than ten percent.
In the art to which this invention relates, the problems of operating batch-type centrifuges with their less than perfectly balanced loads of fine particulate at the very high speeds necessary for drying to extremely low moisture levels have not been solved. That is, in prior methods the constructions used would be unsafe or too expensive for use at the high production rates and at the very high speeds necessary to dry fine particulate to very low moisture levels for practical costs. In addition, the prior art has not addressed the problems of cutting out the fine dried particulate at higher speeds on a dynamic suspension system capable of safe and economical operation.
By way of example, the coal industry has an urgent need for an improved means for drying coal fines smaller than 100 mesh size in economical manner with minimal pollution and safety problems. Prior commercial centrifuges for this service fall into three principal catagories:
(1) Solid bowl decanters with screws for advancing the solids through the bowls;
(2) Screen bowl centrifuges with screws for advancing the solids through the bowls; and
(3) Batch centrifuges, similar to that shown in U.S. Pat. No. 2,271,493, which receive moist particulate at low speeds, raise the bowl speed to a higher speed for drying, and then slow down again for removal of the dried solids. Some of the prior batch-type centrifuges have crude resilient suspension means, U.S. Pat. No. 3,275,152 for example, but they have been unsuitable for the very high speeds and high production rates needed to economically dry very fine coal.
None of these three types of existing centrifuges can be used to obtain a high enough gravity level to dry sub 100 mesh size coal to below twenty to thirty percent surface moisture. Furthermore, the screen bowl centrifuges lose most of the coal of less than 325 mesh size through the screen. Consequently coal cleaning plant operators, who want their fine coal dried to below twenty percent moisture, are left with the choice of using thermal dryers or press-type dryers. Both of these are expensive. Press-type dryers cannot dry very fine coal below fifteen to twenty percent surface moisture. Thermal dryers, although unsafe and potentially environmentally pollutant, can dry fine coal below ten percent surface moisture; however they cannot handle very fine coal unless it is premixed with coarse coal, and the dried coal fines are dusty and will blow away during transportation.